Don't assert in wxDataViewCtrl::ItemDeleted() if item doesn't exist.

It seems that it might be valid to delete the items that the GUI control
doesn't know anything about, e.g. this could happen when deleting a child of a
collapsed node in a tree model. So remove the asserts which were triggered in
this case as there doesn't seem to be any way to avoid them with the current
code.

Closes #11802.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65060 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775